As set forth in my prior U.S. Pat. No. 4,231,770, "baghouses" containing a multiplicity of tubular filter bags are utilized in industrial applications for filtering and cleaning large quantities of air to be exhausted to the atmosphere, wherein entrained particulate material is removed from the air stream prior to it being exhausted. In such baghouses the particulate laden air is directed to and through, the filter bags with the particulate material being filtered and retained on the air contacted surfaces of the filter bags. Baghouses are arranged in two basic manners. They may be arranged as described in my prior, U.S. Pat. No. 4,231,770, wherein the air being filtered is directed from the exterior to the interior of filter bags with the filtered particulate material being retained on the exterior surface of the filter bags. In that arrangement a reverse purge of air may be periodically pulsed through the interior of the filter bags in order to dislodge and remove the trapped particulate material from the exterior surface of the filter bags and thereby at least partially restore the filtering efficiency of the bags.
Alternatively, baghouses may be arranged wherein the air being filtered is directed from the interior to the exterior of the tubular filter bags with the filtered particulate material being filtered and retained on the interior surface of the filter bags. In this internal collection arrangement, the bags are cleaned and renewed by periodically mechanically shaking the bags and/or by periodically directing a reverse flow of cleaning air through the bags from without to within them in order to dislodge and remove the trapped particulate material from the interior surfaces of the filter bags. This basic arrangement is shown in U.S. Pat. No. 3,241,297, for example.
A baghouse with this internal collection arrangement can be quite effective in removing particulate material from large volumes of air. Such baghouses are frequently used at electricity generating plants to remove combustion contaminants from air streams prior to the air being released to the atmosphere.
Effective filtration by such a baghouse arrangement, however, is quite costly. Banks of hundreds and even thousands of filter bags are required to effectively filter the large quantities of air typically encountered. And while these filter bags may operate effectively and efficiently when they are new and first put into service, they become increasingly clogged with particulate material during use, notwithstanding the periodic cleaning cycles mentioned above. Specifically, the particulate material tends to build up unevenly along the length of the filter bag and the normal cleaning efforts do not meet the uneven buildup problems presented. The result is a filter bag with uneven air flow along its length which has less than optimum filtering characteristics.
As this clogging progresses the power required to pass the air through the baghouse increases. Also the filtering abilities of the bag deteriorate and the bag itself is prematurely abraded in certain areas to the point of destruction by frictional contact with the particulate material.
In order to get a better understanding of these problems it is instructive to consider in detail the features and operational characteristics of these filter bags as utilized in internal collection baghouses. Such bags are typically cylindrical in shape and some twelve inches in diameter and twenty to forty feet in length. They may be made of various textile materials or inorganic fibers depending on the heat and corrosion conditions present. The bags are usually installed in the baghouse in multiple rows in tensioned and spaced-apart relationship extending vertically with an open bottom and a closed top. In such an installation dirty air containing particulate material is introduced into the interior of the bag at its bottom opening from where the air passes through the bag material along the length of the bag, thereby leaving entrained particulate material along the interior surface of the bag. Circumferential rings are typically positioned along the length of the bag at spaced intervals to maintain the bag in generally open condition. Otherwise, when reverse air flow is used to clean the surfaces of the bags, the bags tend to collapse and essentially close thus detracting from the cleaning action. If the bags are collapsed during reverse flow cleaning, upon completion of the cleaning cycle and repressurization of the system, the abrupt reintroduction of system air to the interior of the collapsed bags tends to suddenly blow them out and rupture them. These problems are alleviated but not truly solved by use of the rings.
Also, during filtering when the dirty air is constantly entering the bottom open entrance end of the bag and passing through the length of the bag, the portion of the bag adjacent to the bottom open end is constantly being contacted by quantities of air equal to the effective volume of the entire bag along with accompanying quantities of abrasive contaminants. This continuous passage of large volumes of air with contaminants at an accompanying relatively high entrance velocity results in the bag portion adjacent the bottom open end being constantly subjected to destructive forces which unduly wear the bag in this area to the point of premature bag failure.
Furthermore, during filtering operations the effective filtering action of the interior of the bag is theoretically equal along its length, there being a theoretically equal air flow along the length of the bag. In actual practice, however, this theoretical concept breaks down quickly after initial use of a new bag. Specifically, during filtering operations the particulate material entrained in the air in the interior of the bag tends to stratify along the length of the bag according to particle type, size and density with particulate particles accumulating in various areas along the interior length of the bag as opposed to a theoretically uniform distribution. This results in certain interior areas of the bag becoming more clogged than others. Notwithstanding the periodic cleaning mentioned above, during actual operation such localized clogging becomes progressively greater and even more localized. Indeed, the usual cleaning efforts often tend to aggravate the localized nature of the clogging rather than alleviate it. The ultimate result is a filter bag operating at much less than its desired filtering efficiency.
Up to now there have been no real solutions for these problems. The only alternatives have been to replace the prematurely failed bags and to overdesign such baghouse systems and supply them with substantially more bags than are theoretically needed in order to compensate for drops in bag filtering efficiency during use. Such measures have been unduly costly and have not solved the real problems.